OBJECTIVES: The overall objective of these ongoing investigations has been the elucidation of fundamental biological mechanisms involving the formation, propagation, and genetic control of antibiotic resistance plasmids and other extrachromosomal elements --- and modification of these processes as a result of the informaton obtained. OBJECTIVES SET DURING THE CURRENT YEAR: (1) Continued study of transposable genetic elements and other genetic segments of bacterial plasmids carrying antibiotic resistance determinants, (2) investigation of site-specific genetic recombination in vivo mediated by restriction endonucleases and other site-specific recombinational systems, (3) investigation of cryptic endonucleases detectable in vitro in cell extracts but not by in vivo phenotypic expression, (4) investigation of expression of plasmid genes in heterospecific prokaryotic environments.